Don't stop asking
You absolutely must get those customer emails. Forget that you may not have had much success in the past. Remember that there was a time when it was very difficult to get cell phone numbers, yet today it's automatic.
Email is moving that direction rapidly. You don't need gimmicks or contests. Just ask with confidence like you are asking for their phone number and you will likely get the vast majority to cooperate.
Shops which have made it a regular practice have most of their customer's emails...some with over 90%. That makes fast, low cost marketing a snap!